Per-window commands

### What is the problem or limitation you are having?

If you want different menus for different windows, you currently need to detect when the active window changes and alter `App.commands` manually. This most obviously affects DocumentApp, where you might want to enable or disable commands depending on what the user has selected.

But it may also affect other multi-window apps which don't involve "documents" per se. For example:
* In Chrome, the Tab menu contains a list of tabs for the current window only.
* An app to control a robot might provide separate windows for different components of the robot, with content showing information about the component, and different menus for different component types.

### Describe the solution you'd like

Add a `Window.commands` property, which would work as follows:

* On macOS, the app menu would be a union of the commands from the App and the currently-active Window.
* On GTK and WinForms, each window’s menu would be a union of the commands from the App and that window.

Since each window has its own toolbar, this implies that toolbar commands should be automatically added to `Window.commands`, rather than `App.commands` at present.

Start by tracing the existing App.commands and toolbar command behavior, then compare the Window APIs across macOS, GTK, and WinForms. Done means Window.commands supports per-window menus, with the specified App union behavior on macOS and GTK/WinForms, and toolbar commands belong to the appropriate window.

Written by the indexing model from the issue text.